Functionalized lower hydrocarbons, such as isobutylene, propylene, and acetone, are of significant interest for industrial and chemical applications.
Isobutylene, also known as isobutene or 2-methylpropene, is a hydrocarbon of significant interest that is widely used as an intermediate in the production of industrially important products, including para-xylene, jet fuel blendstocks, gasoline oxygenates, isooctane, methacrolein, methyl methacrylate, and butyl rubber. Methods for the conversion of isobutylene into these products are described in U.S. Pat. Nos. 8,193,402, 8,373,012, 8,378,160, 8,450,543, 8,487,149, and 8,546,627, as well as U.S. Patent Application Publication Nos. 2010/0216958, 2011/0087000, and 2012/0171741, each of which is herein incorporated by reference in its entirety for all purposes.
Historically, isobutylene has been obtained through the catalytic or steam cracking of fossil fuel feedstocks. With the depletion of fossil fuel resources, alternative routes for synthesizing isobutylene have been evaluated. In recent years, isobutylene has been generated from the dehydration of the bio-based commodity chemical, isobutanol. See U.S. Pat. Nos. 8,193,402, 8,373,012, 8,378,160, 8,450,543, 8,487,149, and 8,546,627, each of which is herein incorporated by reference in its entirety for all purposes.
Propylene, also known as methylethylene or propene, is a hydrocarbon of significant interest that is widely used as an intermediate in the production of plastic polypropylene which is used throughout industry in the manufacture of films, packaging, caps, and closures. Methods for the conversion of propylene into these products are described in U.S. Pat. Nos. 3,364,190, 7,067,597, 3,258,455, each of which is herein incorporated by reference in its entirety for all purposes.
Historically, propylene has been obtained through the catalytic or steam cracking of fossil fuel feedstocks. With the depletion of fossil fuel resources, alternative routes for synthesizing propylene have been evaluated. In recent years, propylene has been generated from olefin metathesis, also known as disproportionation, in which reversible reactions between ethylene and linear butenes results in the breaking of double bonds followed by reforming to propylene. In addition, propane dehydrogenation converts propane into propylene and byproduct hydrogen. See Patent references US2004/0192994 and WO 2011/136983, each of which is herein incorporated by reference in its entirety for all purposes.
Acetone is a functionalized hydrocarbon of significant interest that is widely used as an intermediate in the production of industrially important products, for example, methyl methacrylate and bisphenol A, as well as a solvent for cleaning purposes. Methods for the conversion of acetone into these and other products are described in U.S. Pat. Nos. EP0407811A2, US5393918, US5443973, EP1186592A1, EP0598243A2, US5434316A, US5210329, US5786522A each of which is herein incorporated by reference in its entirety for all purposes.
Historically, acetone has been obtained directly or indirectly from propylene. Approximately 83% of acetone is produced via the so-called cumene process. As a result, acetone production is tied to phenol production. In the cumene process, benzene is alkylated with propylene to produce cumene, which is oxidized by air to produce phenol and acetone. Other processes involve the direct oxidation of propylene (Wacker-Hoechst process), or the hydration of propylene to give 2-propanol which is oxidized to acetone. Acetone has been previously produced, and continues to be produced, in small quantities using the acetone-butanol-ethanol (ABE process) fermentation process with Clostridium acetobutylicum bacteria.
Bioethanol is also a significant commodity chemical product. With the increased availability and reduced cost of bioethanol, researchers have explored bioethanol as a feedstock for making a variety of downstream hydrocarbons, including the aforementioned hydrocarbon building blocks, isobutylene, propylene, and acetone. Until very recently, a process for the direct conversion of ethanol to isobutylene or propylene had not been described.
In 2011, however, Sun et al. disclosed a method utilizing a nanosized ZnxZryOz mixed oxide catalyst prepared by a carbon template method for the selective conversion of ethanol to isobutylene with a carbon selectivity of 55% (83% of the maximum theoretical yield) from ethanol. In that reference, low levels of propylene have been detected, but not in industrially relevant yields. See Sun et al., 2011, J. Am. Chem. Soc. 133: 11096-11099, which is herein incorporated by reference in its entirety for all purposes. Utilizing a catalyst containing a 1:10 ratio of zinc to zirconium, Sun and colleagues were able to achieve isobutylene yields as high as 83% from ethanol fed at a relatively low molar concentration (0.6%) with less than 5% yield to propylene. Later results published by the same group demonstrated that increasing the molar concentration of ethanol in the feed stream beyond 0.6% dramatically reduces selectivity to isobutylene. Indeed, Liu et al. show that when the ethanol molar concentration in the feed stream increased from 0.6% to 11.9% for a given residence time, the isobutylene yield dropped from 85.4% to 8.2%, which suggests that further increasing the ethanol molar concentration beyond 11.9% would be expected to further reduce the isobutylene yield as well as propylene yield. Liu et al also demonstrated that increasing residence time enabled an increase in the ethanol molar concentration in the feed stream to a maximum 8.3 mol % while still resulting in isobutylene yields of 70-80% of theoretical. See Liu et al., 2013, Applied Catalysis A 467: 91-97, which is herein incorporated by reference in its entirety for all purposes. Accordingly, a process to convert ethanol at high molar concentrations is necessary for the conversion process.
In 2012, Mizuno et al. described the use of indium-oxide (In2O3) catalysts to produce propylene and isobutylene with a sum carbon selectivity of 58.1% (34.1% to propylene and 24% to isobutylene) from ethanol in the absence of exogenously added hydrogen. See Mizuno et al., 2012, Chemical Letters 41: 892-894, which is herein incorporated by reference in its entirety for all purposes. While the teachings of Sun et al. and Mizuno et al. make the direct conversion of bioethanol to isobutylene and/or propylene possible, enhancing the selectivity to these functionalized lower hydrocarbons beyond levels previously achieved (˜55-58% carbon selectivity) can help reduce production costs for bioethanol-derived hydrocarbons. In addition, the methods of Sun et al. and Mizuno et al. are less than optimal because they either utilize a carbon template method for catalyst preparation (Sun) or rely on an expensive element, indium, which is not readily available on a large scale (Mizuno). Accordingly, a more industrially relevant catalyst is necessary for the conversion process.
Previous methods for conversion of ethanol to acetone are disclosed by Murthy et al, 1988, J. Catalysis, 109: 298-302, incorporated herein by reference in its entirety for all purposes, in which a calcium oxide, zinc oxide, or manganese promoted iron oxide catalyst was used. Murthy and colleagues were able to achieve acetone yields as high as 83% of theoretical from ethanol feed at relatively low molar concentrations of ethanol (10 mol % ethanol or 22 wt % ethanol in water). Increasing the ethanol molar ratio to 33% (56 wt % ethanol in water) resulted in only trace amounts of acetone formation. Additionally, the conversion of ethanol to acetone is disclosed by Nakajima et al, 1987, J. Chem Soc, Chem Comm., 6: 394-395, incorporated herein by reference in its entirety for all purposes, in which mixed metal oxides (ZnO, ZnO/CaO, ZnO/Na2O, ZnO/MgO, etc) were used. Nakajima and colleagues were able to achieve acetone yields as high as 91% of theoretical from ethanol fed at low molar concentrations of ethanol (reactor feed comprised of saturated nitrogen generated via bubbling nitrogen through a water/ethanol mixture). Accordingly, both a more industrially relevant catalyst and a process to convert high molar concentrations of ethanol are needed.